Carbon and Nitrogen Stable Isotope Ratios of Subtidal Benthic Invertebrates in an Estuarine Mangrove Ecosystem (Andhra Pradesh, India)

In order to assess the relative trophic importance of mangrove litterfall and aquatic primary production in the mangrove creeks of the Coringa Wildlife Sanctuary (Andhra Pradesh, India) and the adjacent semi-enclosed Kakinada Bay, carbon and nitrogen stable isotope ratios were determined in a variety of benthic invertebrate species collected at 22 sites during the pre-monsoon period (May–June) of 1997 and 1999. 13C values showed little interspecific variation at any given location, but there was a distinct spatial gradient in consumer 13C values of about 7‰, with more depleted values in the mangrove creeks ( 23·6 0·6‰), and gradually increasing in the mangrove outlets ( 21·5 0·9‰), a relatively restricted zone in the south-eastern part of Kakinada Bay adjacent to the mangroves ( 18·8 0·8‰), and the central and northern part of the Bay ( 16·7 1·4‰) which opens into the Bay of Bengal. This gradient is much larger than that observed during a previous study in suspended organic matter (maximum about 2·7‰) and during this study in sediment organic matter (about 1·5–2·5‰). The observed carbon stable isotope ratios thus suggest a marked selectivity of the benthic invertebrate community for pelagic and benthic microalgal food sources and indicate that mangrove-derived and other terrestrial carbon is not a significant food source for benthic invertebrate communities in this ecosystem during the pre-monsoon period. Furthermore, 13C values of sediment organic matter (SOM) suggest that terrestrial carbon is not a major contributor to the SOM-pool in this ecosystem. Evidence for seaward migration of Penaeid prawns was provided by some individuals caught in the North Bay which displayed low 13C values of characteristic of fauna found in the mangrove creeks or outlets. Nitrogen stable isotope ratios were found to be a useful indicator of trophic level, even though there remained some overlap between 15N values of presumed low and higher trophic levels. Benthic invertebrates showed a 15N gradient of about 3·2‰ between the mangrove creeks and the Central and North Bay whereas sediment 15N values showed a smaller spatial gradient of about 1·6‰. This gradient which is hypothesized to reflect differences in inorganic nitrogen sources and availability.